NPT & NPTF Fittings / Thread – Identification & Installation – Heads Up for Hosers

NPT & NPTF Fittings / Thread – Identification & Installation – Heads Up for Hosers


Hi, I am James and this is another episode
of Heads Up for Hosers. In this video we will be discussing National
Pipe Fittings. These fittings include NPT, NPTF, and NPSM. We will go into more detail later about what
these stand for. First we are going to talk about the most
basic Pipe threads, NPT commonly referred to as National Pipe Tapered. NPT threads were originally designed for water
piping applications in low pressure situations. NPT creates a seal by a wedging action. It leaves a gap between the root and the crest
as seen in this graphic This gap creates a possible leak path. Because of this wedge design, these fittings
were never intended to be a connection that would be taken apart on a regular basis. Each time the connection is put back together the wedge must be driven deeper to create a seal. Soon there is no wedge left due to wear. To gain a proper seal in NPT fittings you
need a sealant compound or PTFE. (Polytetrafloroethylene tape). This is because of the gap created by the
lower quality tapered threads. When using a thread tape, you have to make
sure to actually go in the direction of the threads with the tape so as to not jam up
the thread tape when you are putting it into a female NPT fitting. This lower quality thread, caused by rolling,
creates a leak path. Because of this leak path, these fittings
have been condemned by most fluid power associations in high pressure or temperature situations. For example, the use of NPT in gasoline engines
lead to a multitude of problems. Because the fuel dissolved the pipe sealant
and leakage became a real issue. To fix this problem, NPTF was developed. NPTF stands for National Pipe Tapered Fuel,
also referred to as a Dryseal thread. In all appearances, these two threads are
identical, except for small differences in the thread peaks and the valleys. The connection of a NPTF is of a higher quality
because the threads are machined and not rolled. This creates a close metal to metal seal with
no leakage path as was present in NPT. Just like NPT, the threads are crushed together
to make a seal, but the higher quality thread of the NPTF creates a full connection and
thus no sealant is required. You may be asking now how you would tell the
difference between the two fittings? Really the only way is to notice the quality
differences. Because NPT is a rolled thread it will be
of a lesser quality then an NPTF which is milled. Here you can see the quality difference. National pipe thread comes in a wide range
of sizes here at Gregg’s, we are more accustomed to dealing with dash two to dash thirty two. Something to note is that National Pipe Threads
do not come in a dash ten size. Now your probably asking what a dash size
is. Well, a dash size is the determination of
sizes for hose, tubes, and fitting measurements. This is measured in one sixteen inch segments. In this video series we will deal mostly with
hose and fittings, but note that for tubing the dash size calculation is slightly different. Now here are a couple of hose examples:
So, when dealing with most types of hose, dash four equals four one sixteenth inch segments,
which equals four sixteenths, or equal to one quarter of an inch hose ID. Another example is dash thirty two. This is equal to thirty two one sixteenth
inch segments, which equals 32 sixteenths, or is equal to two inch hose ID for most types
of hose. Now, to measure pipe threads you need to have
an understanding of nominal pipe size as well. Nominal pipe sizes are confusing because they are based on an antiquated method of measuring pipe and fittings. Long ago, when pipes started being made they
were measured from their inside diameter, so a 1 inch pipe measured 1 inch on the inside
and 1.32 inches on the outside. When different applications called for higher
pressures they could not change the external dimensions of the thread, because the pipe
had to work in all the same situations. Instead they opted to make the inside diameter
smaller. This means that in this day and age, a 1 inch
NPT fitting may not measure one inch anywhere on the fitting. But its a mouthful to call a 1 inch fitting
a 1.32 or a one and five sixteenths inch fitting, so instead we opt for its nominal size, or
“in name only”, for simplicity. So, getting back to it, to measure NPT or
NPTF threads you follow the same steps. With a male thread, start by measuring the the OD, or Outside Diameter, of the third thread. Here we see it is one and five sixteenths. The next step for measuring National Pipe
threads is to subtract one quarter of an inch from your OD measurement to get to your closest
nominal pipe size. For example: the male fitting we measured
had an OD of one and five sixteenths. Looking in your identification book, we can see that one and five sixteenths minus one quarter comes close to one inch, which is
the nominal pipe size for this fitting. Based on this we see that this is a dash 16
fitting. To verify this, we need to confirm the nominal
thread size. Here it says one inch dash eleven and a half, where the dash eleven and a half means threads per inch. We will take out our thread pitch gauge, and
find the eleven and a half. Next we will take the thread pitch gauge and
we will lay it on the fitting to confirm that we do indeed have 11 1/2 threads per inch. This means that this is a DASH 16 NPTF fitting. A female NPTF fitting will be measured close
to the same way as the male fitting, but it differs in one way, where you take your initial
measurement from. The female fitting ID, or inside diameter,
is measured from the largest thread, which is located at the end of the fitting. After collecting this measurement the steps
to determine the dash size of the fitting are the same as it’s male counterpart. To start, you would determine the ID, or inside
diameter, of your fitting, here we get one and a quarter inch. Consult your book with your measurement and subtract one quarter inch to come close to the nominal thread size. Here we see again that for this female fitting the nominal thread size is one inch eleven and a half. Using our thread pitch gauge we can put the
DASH eleven and a half gauge next to the threads to confirm that it is in-fact a dash sixteen
fitting. NPT and NPT-F fittings install slightly different. To install an NPT fitting you must first use
pipe dope or thread tape. Once hand tightened, to ensure a leak proof
seal, 3 full turns are required. Mark your male and your female and tighten
away. To install an NPTF fitting you do not need
pipe dope or thread tape because the well milled threads produce a leak proof seal on their own. Instead you should just lightly lubricate
the threads with the oil that will be in your system. Once you have it hand tight you only need
1.5 complete turns to ensure it is fully tightened. Any more and you will reduce the life of the
fitting. Although NPT-F corrected the shortcomings
of NPT it is not without its own problems. Because of the wedging action, it is difficult
to orient on elbows being that you have to tighten the fitting to a certain spec, and
once completed the fitting might not be orientated where you need it to be. Also, because of its wedge design this seal
was never intended to be a connection that would be taken apart on a regular basis. Each time the connection is put back together
the wedge must be driven deeper to create a seal and soon there is no wedge due to wear. Here at Gregg’s, we recommend you do not
reuse these fittings. These are a couple of the reasons why the industry has moved away from NPT and NPT-F fittings. A common problem when identifying NPTF fittings
is their relationship to BSPT or British Standard Pipe Tapered threads. Although we have not touched on these fittings
that much yet it is important to know their relationship when attempting to identify NPTF
fittings. Because BSPT fittings share a lot of the same
design characteristics to NPTF they can often be mistaken for each other and, at certain
sizes, even thread together fairly well. Referring to these size charts we can see
that when dealing with DASH 8 or DASH 12 fittings these two share the same nominal pipe size
and thread pitch, making identifying them very difficult. Truthfully the biggest difference is that
Pipe thread has a 60 degree angle thread angle, where as a BSPT has a 55 degree thread angle. This means that even though they feel like
they thread together well, the difference in thread angle means that a true seal is
never formed and there is a chance of leaking. Because there is no discernible difference
between these fittings it is recommend that you question what equipment this fitting came
off of. Depending on the type of equipment and the
country of origin of said equipment you can have a sense for what type of fitting you
are working with. For example: if the equipment is from a European
manufacturer your fittings will most likely be a BSPT rather than an NPTF. This is a rare circumstance that you will
get to this point, but for a Dash 8 or a dash 12 NPTF fitting it is important to note that this mix up could be a possibility. To solve these short comings such as orientation,
on the next episode of Heads Up for Hosers we will discuss the NPSM thread. On behalf of our employee investors, thank
you for watching this video, and we’ll see you on the next episode of Heads Up for Hosers.

5 thoughts on “NPT & NPTF Fittings / Thread – Identification & Installation – Heads Up for Hosers

  1. that's some nice educational stuff guys! under appreciated video, a year old and all. thanks for the hard work in making this.

  2. At 5:49 in video there is a large internal taper on NPT fitting. Does this have any sealing function or is it just the way it is machined internally.

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